Draw a line down from the middle of the nose (between the nostrils) to the upper lip. This line gets wider as you get towards the upper lip and you then draw it along the upper lip to the corners of the mouth. The more quickly you draw the line the straighter it will be, and the more pressure you apply on the brush, the thicker the line becomes.
If you do not want the colours to mix, be sure to wait for the first layer to dry before applying another colour over it or up against it.